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Full-Text Articles in Physics

Inclusion Of Charge Correlations In Calculations Of The Energetics And Electronic Structure For Random Substitutional Alloys, Duane D. Johnson, F. J. Pinski Oct 1993

Inclusion Of Charge Correlations In Calculations Of The Energetics And Electronic Structure For Random Substitutional Alloys, Duane D. Johnson, F. J. Pinski

Duane D. Johnson

Currently, the coherent-potential approximation (CPA) implemented via the multiple-scattering theory of Korringa, Kohn, and Rostoker (KKR) gives the best first-principles description of the electronic structure for random substitutional alloys. However, the total energy has an important component of electrostatic energy missing, namely, that arising from the correlation of charges with varying atomic environments. We develop a ‘‘charge-correlated’’ CPA method (cc-CPA) which includes (some) local environmental charge correlations within the KKR-CPA method. We investigate the cc-CPA energetics for several alloys and show that the formation energies are in better agreement with experimental results. These calculations show that the excess charge on ...


Resistivity Of The High-Temperature Metallic Phase Of Vo2, Werner W. Schulz, Philip B. Allen, Renate M. Wentzcovitch, Paul C. Canfield Aug 1993

Resistivity Of The High-Temperature Metallic Phase Of Vo2, Werner W. Schulz, Philip B. Allen, Renate M. Wentzcovitch, Paul C. Canfield

Paul C. Canfield

Measurements are reported on the electrical resistivity ρ(T) along the c axis of a single crystal of VO2, from the metal-insulator transition at T=333 K up to 840 K. The temperature dependence is very linear, and a fit to Bloch-Grüneisen theory gives a residual resistivity ρ0=65 μΩ cm and a ratio ρ(840 K)/ρ0=8. With the help of a local-density-approximation band-structure calculation, we further pursue the conventional (Bloch-Boltzmann) interpretation by extracting the electron-phonon coupling constant λ=1.1 and the mean free path l(800 K)=3.3 Å. The short mean free path implies ...


Tables Of Coincidence Orientations For Ordered Tetragonal L Lo Alloys For A Range Of Axial Ratios, Abha Singh, Alexander H. King Jan 1993

Tables Of Coincidence Orientations For Ordered Tetragonal L Lo Alloys For A Range Of Axial Ratios, Abha Singh, Alexander H. King

Alexander H. King

In this paper we develop and apply techniques for computation of CSL, DSCL and step-vector data for grain boundaries in tetragonal materials for a range of axial ratios. This has application to L10 alloys including TiA1, which is a candidate for lightweight high-temperature structural applications. Our results are compared with others and found to be more accurate and complete. We provide data for a wider range of axial ratios than those considered by previous workers. We have also derived equivalent quaternions for tetragonal crystals in tetragonal-crystal coordinates and listed conditions for selecting a unique reduced rotation in tetragonal-crystal coordinates so ...


Nqr Study Of Local Structure And Cooling Rate‐Dependent Superconductivity In La2cuo4+Δ, A. P. Reyes, E. T. Ahrens, P. C. Hammel, J. D. Thompson, Paul C. Canfield, Z. Fisk, R. H. Heffner, J. E. Schirber Jan 1993

Nqr Study Of Local Structure And Cooling Rate‐Dependent Superconductivity In La2cuo4+Δ, A. P. Reyes, E. T. Ahrens, P. C. Hammel, J. D. Thompson, Paul C. Canfield, Z. Fisk, R. H. Heffner, J. E. Schirber

Paul C. Canfield

Preliminary results of structural studies in oxygen‐annealed La2CuO4+δ(δ∼0.03) using 139La nuclear quadrupole resonance (NQR) spectroscopy are reported. Superconducting critical temperatures were found to depend on the rate of cooling through a narrow temperature range near 195 K. Analysis of the 139La NQR spectra reveal that lanthanum atoms in the oxygen‐rich metallic region occupy two sites having different local environments. One site has a structural configuration closely related to the stoichiometric oxygen‐poor compound. The second appears only below 200 K and exhibits a large shift and broad distribution of NQR frequency ν Q . Shifts ...


A Geometrical Rationalization Of The Special Properties Of The 14° [001] Grain Boundary In Yba2cu3o7−Δ, Alexander H. King, Abha Singh Jan 1993

A Geometrical Rationalization Of The Special Properties Of The 14° [001] Grain Boundary In Yba2cu3o7−Δ, Alexander H. King, Abha Singh

Alexander H. King

14° [001] tilt grain boundaries in YBa2Cu3O7−δ have been found to exhibit anomalously good superconducting critical currents, associated with magnetic flux pinning, although no special interfacial structure has heretofore been associated with this misorientation. We present an analysis of the geometry of the grain boundary in question and show that it can correspond to a special interface, within the definition of constrained lattice coincidence, which may lead to new understanding of the superconducting behavior of grain boundaries. Our analysis shows that the tetragonal‐to‐orthorhombic phase transformation may have important effects upon grain boundary structure and properties, indicating new ...